1. Field of the Invention
This invention relates to an optical compensator, and more particularly to an optical compensator sheet having a negative type optical anisotropy in which the refractive index normal to the plane of sheet is smaller than those in the plane of sheet.
2. Description of the Related Art
It is known that when a film of polymer material is stretched in one direction, the optical refractive index in the stretched direction becomes larger than the optical indices in the directions perpendicular to the stretched direction.
For example, when a polycarbonate sheet is prepared, x and y axes are taken in the plane of sheet and z axis is taken in the direction perpendicular to the plane of sheet, the optical refractive indices along the respective actual directions are usually n.sub.x =n.sub.y =n.sub.z. When this sheet is stretched in x direction, the refractive index n.sub.x in x direction, which is the stretched direction, becomes larger than the refractive indices n.sub.y and n.sub.z in y and z directions, which are the directions perpendicular to the stretched direction, n.sub.x &gt;n.sub.y =n.sub.z.
In a liquid crystal display device utilizing homeotropic orientation, optical compensation is required.
When liquid crystal molecules are homeotropically orientated normal to a substrate, the refractive index of the liquid crystal layer in the direction perpendicular to the substrate becomes higher than the refractive indices in directions parallel to the substrate, to show a positive optical anisotropy.
When an incident light ray comes obliquely to the substrate, the incident light ray is influenced by the optical anisotropy of the liquid crystal layer, thus, there occurs light leakage in the "OFF" state. This light leakage limits the viewing angle.
For improving the viewing angle of the liquid crystal display device, there is desired an optical compensator plate having a negative optical anisotropy for compensating the positive optical anisotropy of the liquid crystal layer.
However, it has been not easy to provide an optical compensator plate having a uniform negative optical anisotropy with an optic axis perpendicular to the plane of plate.